People at John F. Kennedy Airport in New York watch a TV broadcast of President Obama announcing the death of Osama Bin Laden early Monday.

Eric Thayer/REUTERS

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Airports and other potential targets of terrorist attack operated Monday in a state of heightened vigilance following the death of Al Qaeda leader Osama bin Laden in a Pakistan firefight with US Navy forces.

Despite concern about possible efforts to retaliate by Al Qaeda or groups aligned with the terror organization, security officials also sought to encourage a business-as-usual atmosphere.

John Brennan, the top counterterrorism adviser to President Obama, said the US government knew of no immediate specific threats.

In the US, the Transportation Security Administration issued a low-key advisory, including this statement: "As always, passengers may notice a variety of security measures at US airports to include the use of physical bag checks, random gate screening, explosives-detection technology, canine teams, and behavior detection officers."

The statement asked travelers to report any suspicious activity to authorities, and Mr. Brennan said US security agencies would ramp up their vigilance against possible attacks.

• New York City authorities briefly shut down a street in Times Square after investigating a suspicious package Monday which turned out not to be dangerous, the city's fire department said.

Denmark's intelligence agency warned about possible backlash attacks, although it said they are likely to be executed by individual and smaller groups outside Europe, especially in Pakistan.

"For militant Islamists, there will be prestige in being the first to avenge him and demonstrate that Islamic militants continue to operate," it said in a terror assessment Monday. "It is [our] assessment that the threat will be especially directed against US interests."

In Germany, which had arrested three suspected Al Qaeda members Friday on suspicion of making a shrapnel-laden bomb to attack a crowded place, there was no visible beefing up of security.

Interior Minister Hans-Peter Friedrich predicted the death of bin Laden will weaken terrorism in the medium to long-term. "[But] we must be clear that terrorist cells working in Europe and beyond exist very autonomously, will continue to exist, and we still have a high threat level in Germany and Europe."